The subject matter disclosed herein relates generally to imaging systems and methods, and more particularly, to systems and methods for selecting parameters for imaging.
Computed Tomography (CT) imaging systems typically include an x-ray source and a detector. As the x-rays pass from the source through the object being imaged, the x-rays become attenuated before impinging upon the detector. The intensity of the attenuated beam radiation received at the detector is responsive to the attenuation of the x-rays by the object, with detector elements producing separate electrical signals responsive to the beam attenuation at the detector element location. These electrical signals may be referred to as x-ray attenuation measurements. Further, the x-ray source and the detector array may be rotated around the object to be imaged such that an angle at which an x-ray beam intersects the object changes. A group of x-ray attenuation measurements, or projection data, from a detector at one gantry angle may be referred to as a “view.” A set of views made at different gantry angles during one revolution of an x-ray source and detector may be referred to as a “scan.” In an axial scan, projection data is processed to construct an image that corresponds to a two-dimensional cross-section or slice of an object being scanned
An image may be reconstructed, for example, using a technique referred to as a “filtered back-projection technique.” This process converts the attenuation measurements from a scan into discrete integers, ranging from −1024 to +3071, called “CT numbers” or “Hounsfield Units” (HU). These HU's are used to control the brightness of a corresponding pixel on a cathode ray tube or a computer screen display in a manner responsive to the attenuation measurements. This integer conversion, or scoring, allows a physician or a technician to determine the approximate density of matter based on the intensity of the computer display.
Certain scanning parameters, such as x-ray tube, or emitter, current (“mA”), x-ray tube supply voltage (“kV” or “kVp”), slice thickness, scan time, or helical pitch (for helical scans) are known to affect the x-ray beam, which in turn affects image quality. A higher x-ray tube current may, for example, improve image quality (e.g., by reducing image noise), but increase the dosage of radiation received by a patient. Lower x-ray tube current levels may lead to streaking or other artifacts in an image. With regard to x-ray tube voltage, conventional CT scanners may provide for several tube voltage stations (for example, discrete stations ranging between 80 kV to 140 kV) that allow a technician and/or physician to select the x-ray tube voltage. However, conventional voltage selection may be mostly responsive to the preference of the physician, and thus typically may lack sufficient guidance. While on one hand higher tube voltage provides for better geometric dose efficiency for larger patients, lower tube voltage has been shown to provide for better contrasts for different types of materials (e.g., contrast-enhanced lesions). Such tradeoffs may not be well known and, thus, the emitter tube voltage selection is generally fixed for a certain type of scan regardless of the patient size or clinical intent.
In the past, CT scans have been performed, for example, with tube currents adjusted to match a patient size and intended use. In terms of tube voltage, however, little or no adjustment may have been made for patient or usage in certain systems. Because of the interaction of tube voltage and current, image quality may suffer and/or unacceptable levels of noise may be present in scans for which tube voltage has been adjusted. Further, selection of tube voltage may be difficult, complicated, or poorly explained, so that the lack of ease of use has resulted in practitioner's often failing to adjust a tube voltage for a particular patient or procedure. Because of the complexity, for example, of interactions between aspects of systems and/or between parameters defining the operation of scanning systems, it is difficult to adjust tube voltage, for example to optimize radiation dose while maintaining tolerable image quality. Therefore, non-optimal usage of a scanning system may result.